This invention relates to an adhesive clad insulating substrate used for producing printed circuit boards by an additive process.
Printed circuit boards are produced by an additive process, for example, by the following steps; drilling of through-holes on an insulating substrate coated with an adhesive containing an electroless plating catalyst .fwdarw. chemical roughening of the surface of an adhesive layer .fwdarw. formation of circuits by electroless plating; or drilling of through-holes on an insulating substrate coated with an adhesive containing an electroless plating catalyst .fwdarw. chemical roughening of the surface of an adhesive layer .fwdarw. electroless plating on the whole surface thereof .fwdarw. formation of a resist on portions other than circuits to be formed .fwdarw. electroplating .fwdarw. removal of the resist .fwdarw. removal of plating on portions other than the circuit portions by quick etching; and the like. When a substrate containing no catalyst for electroless plating is used, it is immersed in a solution containing a catalyst for electroless plating in a suitable stage before the electroless plating.
The adhesive in the printed circuit boards produced by the additive process should satisfy the fundamental requirements that it is susceptible to chemical roughening for tightly adhering a metal conductor circuit, that as a result, it is sufficient in bond strength, and that it has excellent heat resistance (solder heat resistance) enough to withstand soldering at a high temperature for loading electronic parts. The surface of a layer of the adhesive should be chemically roughened before the electroless plating. As an adhesive for this purpose, there have generally been proposed adhesives obtained by using as their main constituent a synthetic rubber which is susceptible to chemical roughening and flexible, for example, acrylonitrile-butadiene rubber, crosslinking it by use of an alkylphenol resin in order to keep the heat resistance, and properly mixing therewith an epoxy resin or the like (for instance, U.S. Pat. No. 3,625,758).
However, adhesives for printed circuit boards by additive process used in electronic machines which have been highly developed more and more in recent years have come to be required to be improved in heat resistance.
For example, electronic parts loaded on a printed circuit board by soldering are often replaced because of troubles in the circuit. In such a case, rectification by manual soldering is conducted by using a soldering iron having a temperature usually as high as 300.degree. C. or higher, and in some cases, 400.degree. to 420.degree. C., and the adhesives in the places loaded with soldered parts is softened, so that a circuit is peeled off and tends to be cut. Therefore, replacement of the parts is very difficult. A reason for such insufficiency in heat resistance of the adhesive is as follows. The cross-linking between acrylonitrile-butadiene rubber and phenolic resin is insufficient, and even when an epoxy resin is mixed, it does not react with the acrylonitrile-butadiene rubber and they cure individually, so that the cross-linking of acrylonitrile-butadiene rubber is not improved, resulting in insufficient heat resistance.